f2fs: introduce f2fs_change_bit to simplify the change bit logic

Introduce f2fs_change_bit to simplify the change bit logic in
function set_to_next_nat{sit}.

Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

diff --cc fs/f2fs/f2fs.h
Simple merge

diff --cc fs/f2fs/node.h
Simple merge

diff --cc fs/f2fs/segment.h
Simple merge